Mapping the neutral atomic hydrogen gas outflow in the restarted radio galaxy 3C 236

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An active galactic nucleus (AGN) releases a vast amount of energy that affects the evolution of its host galaxy through a complex interplay between accretion of matter onto the central supermassive black hole, gas from the interstellar medium (ISM) and star formation activity. In particular, the AGN can cause outflows of ISM gas which could prevent accretion and star formation. In this recently accepted paper, we focus on one aspect of this interaction which is well suited for high-resolution radio observation: the interaction between radio jets and neutral atomic hydrogen gas.

To this purpose, we study the active galaxy 3C 236 which is not only one of the largest known, but shows also signs of recurring AGN activity. The inner 1kpc-region is dominated by a (young) compact steep spectrum radio source, towards which previous WSRT observations revealed a high-velocity and massive outflow of hydrogen gas in absorption. In our new paper, we investigate the outflow in detail with a global very long baseline interferometry (VLBI) array comprising the European VLBI Network (EVN), Very Long Baseline Array (VLBA) and the Arecibo radio telescope which is complemented by a Very Large Array (VLA) observation. The VLA data confirms the presence of the outflow and the excellent high-resolution of our VLBI observation allows us to spatially resolve the outflow. Even though we do not manage to recover all of the outflow seen by the WSRT and VLA, we detect four distinct clouds that have kinematic properties inconsistent with the regular rotation of the majority of the hydrogen gas. The highest velocity cloud reaches a velocity of more than 600km/s. It is possible that "missing" outflow consist of clouds similar to the ones that we detected, but we cannot rule out the presence of a diffuse outflow component.

The image shows several 3D renderings of the hydrogen gas in front of the radio continuum source that we observe with VLBI. The entire animation which also features a great LOFAR image provided by Aleksander Shulevski (paper in preparation) was done with Blender and is available on YouTube: https://youtu.be/6WY9nbVKLGI

Reference: R. Schulz, R. Morganti, K. Nyland, Z. Paragi, E. K. Mahony,

T. Oosterloo, A&A accepted, https://arxiv.org/abs/1806.06653"